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<p>模板声明（<a href="class_template.html" title="cpp/language/class template">类</a>、<a href="function_template.html" title="cpp/language/function template">函数</a><span class="t-rev-inl t-since-cxx14"><span>及<a href="variable_template.html" title="cpp/language/variable template">变量</a></span> <span><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></span></span>）可在任何不是<a href="class.html#.E5.B1.80.E9.83.A8.E7.B1.BB" title="cpp/language/class">局部类</a>的 class、struct 或 union 的<a href="class.html" title="cpp/language/class">成员说明</a>中出现。
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;vector&gt;</span>
<span class="co2">#include &lt;algorithm&gt;</span>
 
<span class="kw1">struct</span> Printer <span class="br0">{</span> <span class="co1">// 泛型函子</span>
    <a href="../io/basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&amp;</span> os<span class="sy4">;</span>
    Printer<span class="br0">(</span><a href="../io/basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&amp;</span> os<span class="br0">)</span> <span class="sy4">:</span> os<span class="br0">(</span>os<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
    <span class="kw4">void</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&amp;</span> obj<span class="br0">)</span> <span class="br0">{</span> os <span class="sy1">&lt;&lt;</span> obj <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// 成员模板</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../container/vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> v <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>,<span class="nu0">2</span>,<span class="nu0">3</span><span class="br0">}</span><span class="sy4">;</span>
    <a href="../algorithm/for_each.html"><span class="kw1568">std::<span class="me2">for_each</span></span></a><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, Printer<span class="br0">(</span><a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
    <a href="../string/basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s <span class="sy1">=</span> <span class="st0">"abc"</span><span class="sy4">;</span>
    <a href="../algorithm/for_each.html"><span class="kw1568">std::<span class="me2">for_each</span></span></a><span class="br0">(</span>s.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, s.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, Printer<span class="br0">(</span><a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">1 2 3 a b c</pre></div></div> 
</div>
<p>在类作用域和外围命名空间作用域中都可以出现成员模板的部分特化，但显式特化只能出现于外围命名空间作用域中。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">struct</span> B<span class="sy4">;</span>         <span class="co1">// 主成员模板</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">struct</span> B<span class="sy1">&lt;</span>T<span class="sy2">*</span><span class="sy1">&gt;</span> <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK：部分特化</span>
<span class="co1">//  template&lt;&gt; struct B&lt;int*&gt; { };      // 错误：全特化</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> A<span class="sy4">::</span><span class="me2">B</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy2">*</span><span class="sy1">&gt;</span> <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span>       <span class="co1">// OK</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">struct</span> A<span class="sy4">::</span><span class="me2">B</span><span class="sy1">&lt;</span>T<span class="sy3">&amp;</span><span class="sy1">&gt;</span> <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span>  <span class="co1">// OK</span></pre></div></div>
<p>若外围类定义也是类模板，则在类体外定义成员模板时，它带有两组模板形参：一组是外围类的，另一组是其自身的：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T1<span class="sy1">&gt;</span>
<span class="kw1">struct</span> string <span class="br0">{</span>
    <span class="co1">// 成员模板函数</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T2<span class="sy1">&gt;</span>
    <span class="kw4">int</span> compare<span class="br0">(</span><span class="kw4">const</span> T2<span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="co1">// 构造函数亦可为模板</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T2<span class="sy1">&gt;</span>
    string<span class="br0">(</span><span class="kw4">const</span> <a href="../string/basic_string.html"><span class="kw1228">std::<span class="me2">basic_string</span></span></a><span class="sy1">&lt;</span>T2<span class="sy1">&gt;</span><span class="sy3">&amp;</span> s<span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/*...*/</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// string&lt;T1&gt;::compare&lt;T2&gt; 的类外定义 </span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T1<span class="sy1">&gt;</span> <span class="co1">// 对于外围类模板</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T2<span class="sy1">&gt;</span> <span class="co1">// 对于成员模板</span>
<span class="kw4">int</span> string<span class="sy1">&lt;</span>T1<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">compare</span><span class="br0">(</span><span class="kw4">const</span> T2<span class="sy3">&amp;</span> s<span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/* ... */</span> <span class="br0">}</span></pre></div></div>
<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0.E6.A8.A1.E6.9D.BF">成员函数模板</span></h3>
<p>析构函数和复制构造函数不能是模板。若声明了可用<a href="copy_constructor.html" title="cpp/language/copy constructor">复制构造函数</a>的类型签名实例化的模板构造函数，则替而使用隐式声明的复制构造函数。
</p><p>成员函数模板不能为虚，且派生类中的成员函数模板不能覆盖来自基类的虚成员函数。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> Base <span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> Derived <span class="sy4">:</span> Base <span class="br0">{</span>
    <span class="co1">// 此成员模板不覆盖 Base::f</span>
    <span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span>
 
    <span class="co1">// 非模板成员覆盖函数可以调用该模板：</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> override <span class="br0">{</span>
         f<span class="sy1">&lt;&gt;</span><span class="br0">(</span>i<span class="br0">)</span><span class="sy4">;</span>
    <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>可以声明具有相同名字的非模板成员函数和模板成员函数。在冲突的情况下（某个模板特化与非模板函数的签名严格匹配），对该名字和类型的使用指代的是非模板成员，除非提供显式模板实参列表。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw1">struct</span> A <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 非模板成员</span>
 
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T2<span class="sy1">&gt;</span>
    <span class="kw4">void</span> f<span class="br0">(</span>T2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 成员模板</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// 模板成员定义</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T2<span class="sy1">&gt;</span>
<span class="kw4">void</span> A<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span>T2<span class="br0">)</span>
<span class="br0">{</span>
    <span class="co1">// 一些代码</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    A<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span> ac<span class="sy4">;</span>
    ac.<span class="me1">f</span><span class="br0">(</span><span class="st0">'c'</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用模板函数 A&lt;char&gt;::f&lt;char&gt;(int)</span>
    ac.<span class="me1">f</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>   <span class="co1">// 调用非模板函数 A&lt;char&gt;::f(int)</span>
    ac.<span class="me1">f</span><span class="sy1">&lt;&gt;</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用模板函数 A&lt;char&gt;::f&lt;int&gt;(int)</span>
<span class="br0">}</span></pre></div></div>
<p><br>
成员函数模板的类外定义必须<i>等价</i>于类内声明（等价性的定义见<a href="function_template.html#.E5.87.BD.E6.95.B0.E6.A8.A1.E6.9D.BF.E9.87.8D.E8.BD.BD" title="cpp/language/function template">函数模板重载</a>），否则它被认为是一个重载。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> X <span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> T good<span class="br0">(</span>T n<span class="br0">)</span><span class="sy4">;</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> T bad<span class="br0">(</span>T n<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">struct</span> identity <span class="br0">{</span> <span class="kw1">using</span> type <span class="sy1">=</span> T<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// OK：等价声明</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> V<span class="sy1">&gt;</span>
V X<span class="sy4">::</span><span class="me2">good</span><span class="br0">(</span>V n<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> n<span class="sy4">;</span> <span class="br0">}</span>
 
<span class="co1">// 错误：不与 X 内的任何声明等价</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
T X<span class="sy4">::</span><span class="me2">bad</span><span class="br0">(</span><span class="kw1">typename</span> identity<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">type</span> n<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> n<span class="sy4">;</span> <span class="br0">}</span></pre></div></div>
<h3><span class="mw-headline" id=".E8.BD.AC.E6.8D.A2.E5.87.BD.E6.95.B0.E6.A8.A1.E6.9D.BF">转换函数模板</span></h3>
<p>用户定义的<a href="cast_operator.html" title="cpp/language/cast operator">转换函数</a>可以是模板。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
    operator T<span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 转换到指向任何类型的指针</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// 类外定义</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
A<span class="sy4">::</span><span class="me2">operator</span> T<span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="kw1">return</span> nullptr<span class="sy4">;</span><span class="br0">}</span>
 
<span class="co1">// 对 char* 的显式特化</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>
A<span class="sy4">::</span><span class="me2">operator</span> <span class="kw4">char</span><span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="kw1">return</span> nullptr<span class="sy4">;</span><span class="br0">}</span>
 
<span class="co1">// 显式实例化</span>
<span class="kw1">template</span> A<span class="sy4">::</span><span class="me2">operator</span> <span class="kw4">void</span><span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    A a<span class="sy4">;</span>
    <span class="kw4">int</span><span class="sy2">*</span> ip <span class="sy1">=</span> a.<span class="me1">operator</span> <span class="kw4">int</span><span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 显式调用 A::operator int*()</span>
<span class="br0">}</span></pre></div></div>
<p>在<a href="overload_resolution.html" title="cpp/language/overload resolution">重载决议</a>中，<a href="lookup.html" title="cpp/language/lookup">名字查找</a>不会找到转换函数模板的特化。取而代之的是，所有可见的转换函数模板都会受到考虑，且每个<a href="template_argument_deduction.html" title="cpp/language/template argument deduction">模板实参推导</a>所产生的特化（对于转换函数模板有特殊规则）都会得到使用，如同被名字查找所找到一样。
</p><p>派生类中的 using 声明不能涉及来自基类的模板转换函数。
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<p>用户定义转换函数模板不能有推导的返回类型
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
  operator <span class="kw4">auto</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">10</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// OK</span>
  <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> operator <span class="kw4">auto</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">42</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// 错误</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<h3> <span class="mw-headline" id=".E6.88.90.E5.91.98.E5.8F.98.E9.87.8F.E6.A8.A1.E6.9D.BF">成员变量模板</span></h3>
<p>变量模板可声明于类作用域内，该情况下它声明静态数据成员模板。细节见<a href="variable_template.html" title="cpp/language/variable template">变量模板</a>。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td></tr>
</table>
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> 应用于
</th>
<th> 出版时的行为
</th>
<th> 正确行为
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue1878">CWG 1878</a>
</td>
<td> C++14
</td>
<td> 技术性允许 operator auto
</td>
<td> 禁止 operator auto
</td></tr></table>

<!-- 
NewPP limit report
Preprocessor visited node count: 1250/1000000
Preprocessor generated node count: 4988/1000000
Post‐expand include size: 34930/2097152 bytes
Template argument size: 8276/2097152 bytes
Highest expansion depth: 15/40
Expensive parser function count: 0/100
-->

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